1. Field of the Invention
The present invention pertains to a system for finding discontinuities in generally inaccessible cables and more specifically to a system for very accurately finding faults or splices in underground power cables and the like.
2. Prior Art
It has always been very difficult to find faults and splices in inaccessible cables such as underground power cables. Further, it is extremely important to be able to accurately determine the position of the fault or splice because much time and labor is expended in digging and uncovering portions of the cable to actually gain access to the fault or splice for repair purposes.
Current art attempts to locate faults and splices in a cable by using a time domain reflectometer (TDR) also called a cable-radar. A TDR is an instrument that transmits a short electrical pulse into one end of a cable and then receives a return pulse that is reflected from the cable fault or splice point.
A TDR operates on the principle that any discontinuity in the electrical impedance of the cable will reflect part of the transmitted pulse energy. Low resistance faults, opens, shorts, and cable splices form such an impedance discontinuity and can therefore be detected using a TDR.
The TDR only measures the electrical distance to the fault. The electrical distance is converted to physical distance from knowledge of the approximate propagation velocity of the TDR pulses. This physical distance is then found by tracking the cable path using a tone generator and precisely measuring distance from the TDR using a tape measure. This is a very time consuming process because the exact path of the cable must be accurately traced and measured to obtain a reasonable indication of the fault location.
For locating high resistance faults, the TDR is combined with a device known as a "thumper". A thumper is a piece of equipment that applies high voltage dc pulses into a sectionalized cable run. The thumper is used to induce arcing at the high resistance fault, thus producing a low resistance fault for a short time during the discharge arc period. The location of this low resistance discharge arc can then be determined either by using a TDR or by timing the discharge pulse transients that travel back and forth between the fault and the thumper.
The TDR measures actual electrical distance to the fault which is usually quite different than the physical measurement of the cable path above ground. Some of the reasons for this difference is the meandering of the cable, both in vertical and horizontal directions, inability to accurately follow the cable because of obstructions and the like, compounded measurement errors in the tone generator tracking procedure, etc. This difference is the main reason that a TDR is limited to prelocation rather than the pinpointing of a fault.
To pinpoint the fault, one or two crew members go to the TDR indicated approximate fault area. A third crew member energizes the thumper while the crew members at the fault area listen for the audible sound of the high voltage discharge arc. If the crew members at the fault area cannot hear the discharge "thump" they often increase the voltage on the thumper until the thump becomes audible. The combination of excessive thump pulse time and excessive dc voltage on the cable damages the polyethylene cable insulation and can cause the cable to fault prematurely in the future.
A further problem with TDR is that extremely long cables and large numbers of discontinuities drain the energy of the initial pulse. TDR signals are attenuated as they propagate down the cable under test. As a result, equal amplitude discontinuities appear smaller at longer distances down the cable than they do at shorter distances. An operator can be misled into interpreting a small reflection at a long distance down the cable as originating from a minor discontinuity and possibly dismissing it as being insignificant. In reality, however, the discontinuity may be quite significant and important.
In addition to cable attenuation, short range discontinuities reflect significant energy, thus leaving less energy available to illuminate discontinuities further down the cable. This reduced incident energy on distant discontinuities again can confuse the operator as to their true amplitude and significance.
Another problem in using TDRs to test cables is the confusion that arises from higher order reflections. Higher order reflections are defined here as sensed energy pulses that are produced by multiple reflections from a single discontinuity. These multiple reflections can be falsely interpreted as additional cable discontinuities when in fact they are simply repeats of legitimate cable discontinuities.